Global Positioning Systems

GPS
The Global Positioning System (GPS) is a world-wide system formed from a series of 24 satellites and ground stations located around the United States and across the world. GPS uses these satellites to triangulate the position of the receiver on the Earth's surface. Triangulating is basically measuring the distance from at least three of the satellites to your receiver. The more satellites you have available, the more accurate your reading will be; fewer satellites means less accuracy.

Accuracy
GPS units vary in range and price. Any user can go and buy a GPS unit at your nearest shopping center or sporting goods store. Accuracy of the GPS unit varies coincidentally with the price of the unit. A low-cost unit, which may have an accuracy of 30-feet or so, is good for the casual user and is used by many people for hunting, fishing, and geocaching.

The opposite end of the spectrum is the survey-grade GPS unit, one whose accuracy can put you within a centimeter of its actual location on the earth. These GPS units are used by survey crews and any other faction that needs a great amount of accuracy in their data collection.

Types of GPS
The City of Dubuque is currently using several types of GPS. The types of units vary between departments, and the accuracy of each unit reflects the accuracy needed for each project.

The City currently has two Trimble Nomad GPS units. These units generally give users an accuracy of 3-5 meters, but have other unique features that make them a great tool in the field. Both units are equipped with a built-in camera, which allows staff to take pictures of the features they are GPSing in the field. One of these units also has a built in scanner, which allows the user to scan bar-codes directly into a GIS database.

Public Works, Engineering, and Water Distribution are taking advantage of Trimble GeoXH GPS units. These units, when using post-processed data, have accuracy of 1-foot. Water Distribution has taken their GeoXH a step further, and are also using a Zephyr antenna and pole with their GPS unit. This gives Water Distribution a 10-cm accuracy on their features in the field - an accuracy they demand with the high number of features in their database.

Finally, Engineering has a survey-grade GPS unit. This unit has the highest on-the-fly accuracy of all our GPS units, but does not allow the user to collect any attribute data about the features being GPS'd.

Besides the GPS location of the features, each of these units (less the survey-grade unit in Engineering) allows the user to collect attribute data about the feature they are GPSing. This might include what type of feature it is (manhole, hydrant), the diameter of a feature, the condition of a feature, or even the day a feature was serviced or cleaned.